1. Field of the Invention
The present invention relates to an optical modulation device for effecting modulation by causing light to undergo a desired diffraction phenomenon.
In particular, the present invention relates to an optical modulation device for effecting modulation by changing the characteristics of a diffraction grating composed of a specific substance and a substance whose refractive index can be varied so as to control the refractive index of the substance whose refractive index can be varied.
2. Related background Art
One conventionally well-known optical modulation device is a liquid crystal display device of the so-called TN (twist nematic) type which comprises a pair of polarizing plates disposed such that their polarizing directions are orthogonal to each other having a device disposed between the pair of polarizing plates having a liquid crystal sealed therein. An orientation treatment is given so that the polarizing directions become perpendicular to the respective opposed surfaces of a pair of transparent substrates to effect switching of the state of orientation of the liquid crystal between a twisted state and a state of being oriented perpendicular to the substrate surfaces. Since this type of display device has a simple arrangement and it is easy to drive the same, it is used in a wide range of applications. However, since the transmission and attenuation of a luminous flux is effected by two polarizing plates, the rate of the transmittance at the time of color extinction (i.e., at the time of light transmission) is poor. Therefore, it cannot be said that this type of optical modulation device is favorable in terms of the efficiency of utilizing a luminous flux.
Another display device of the same type which makes use of a liquid crystal is called a guest-host mode liquid crystal display device, which is used after mixing pigments in the molecules of the liquid crystal. With this display device, the rate of transmittance at the time of color extinction is at best 75% or thereabout due to the inclusion of pigments.
Meanwhile, a display device or a variable color subtractive filter element which combines a reflection or transmission-type phase diffraction grating and a liquid crystal is disclosed, inter alia, in Japanese Patent Examined Publication No. 3928/1978 and U.S. Pat. No. 4,251,137. Although the devices disclosed therein excel in the efficiency with which they utilize a luminous flux, the device disclosed in Japanese patent Examined Publication No. 3928/1978 merely exhibits a decoration effect, so it has been unsatisfactory as a display device for displaying characters and images or as an optical modulation device for effecting transmission and cutting off of a luminous flux. In addition, the variable subtractive color filter element disclosed in U.S. Pat. No. 4,251,137 is arranged such that a diffraction grating is formed by an isotropic material in such a manner that the directions of orientation become perpendicular to each other with respect to a pair of opposed substrate surfaces. A liquid crystal is disposed between these substrates, and its refractive index is changed by controlling the state of orientation of the molecules of the liquid crystal, thereby changing any difference of the refractive indices between the liquid crystal and the substance constituting the diffraction grating so as to make the spectral transmittance characteristics variable. Thus, this variable subtractive color filter element excels in the efficiency with which it utilizes a luminous flux and exhibits high performance as a variable color filter. With this type of optical modulation device using a conventional diffraction grating, however, since the liquid crystal is orientated homogenously, an incident beam of light is diffracted in a static state, so that, when it is actually used as a display device, it has been necessary to constantly apply a voltage or the like in order to maintain a non-display state, i.e., one allowing all the incident luminous fluxes to be transmitted therethrough (i.e., a normally closed state). Accordingly, these prior art display devices have experienced certain problems such as the large amount of power that is consumed and a substantial decline in reliability.